The horn is not working - 2002 Mitsubishi Lancer

Disconnect the horn wiring at the horn. Check for voltage at the horn when the horn 'button' is pushed. If there is voltage at the horn, the problem is the horn has died. Remove and replace. No voltage at the horn is most likely related to the horn 'button' but could be wiring. Could also be corroded connections at the horn wiring.
Just had to replace the horn on a 2011 Chevy.
A 1930 Model A horn still works though.

The horn relay primary receives + full time and is actuated by receiving ground (-) from the horn button. Check ground continuity at the horn button (at the steering wheel) to make sure it is working first (most likely culprit). If the horn button is working, check to see if the horn relay is working (second most likely culprit). To test the horn relay, provide chassis or battery ground to the (-) primary terminal of the horn relay and see if the horn honks. If it does, that will tell you that the problem is in the wiring between the steering wheel and the relay, most likely a disconnected wire at the steering column.
The only other components in the system are the fuse for the horn circuit (if you have 12v at the horn relay, that is fine), and the horn itself. My bet is on the horn button pad itself.

WITH KEY ON EVERY THING ELSE OFF PUSH THE HORN YOU SHOULD HEAR A SMALL CLICK THAT THE HORN RELAY IF SO TEST FOR POWER AT THE HORN MAKE SURE IT IS STILL CONNECTED IF POWER IS GOOD REPLACED HORN

first run a live wire from the battery to the horn connection to see if the horn works,now some older cars have a permanant feed to the horn and then the return to the horn switch so have a look to see how many wires on the horn ,if two wires then one side of the horn to the + and the other to the earth - and see if it works

There will only be one fuse for the horn. Find and check the horn relay. If that works check the wiring and the horn switch..

Could be bad horn or relay or the horn'button'.Start at the horn with a test light.Have someone push horn button and check for power at the horn.If no power then trace wires to relay,check for power cominginto relay.If power is going in but not out to horn relay is problem.etc. Some horns have a lttle knob you can turn to adjust them and get them working

The problem can be with the horn pad, the horn relay or the horn itself. Find your horn relay, it should be in the engine compartment fuse box. Remove the relay and test for power at the legs. there should be two legs that have power. If there is power there, take the cover off the relay and plug it back in. Squeez the relay points together and see if the horn works. If the horn still does not work, it is probably the horn itself. If it does work it is probably the horn pad, but you don't want to mess with that because of the air bag. Let a pro do it.

All of this is predicated on the fact that you have checked all the fuses.

Hope this helps
Rich

Maybe horn became unplugged. Put 12 v power to horn make sure horn works. Put test light on terminal on horn have someone press the horn button, if light comes on and horn does not blow you may need new horn. if light does not come on you may have bad contact at horn button, blown fuse, broken wire, bad horn relay or possibly just a bad ground.

Unplug the wire to the horn and make sure you have the horn grounded really good. Take a hot wire and run it from the battery to the horn. touch the hot wire to the end of the horn and see if it works. If it does not work then you need to replace the horn. If the horn works then take a test light and hook it up to the horn wire and have someone push the horn down. you should have a hot wire. If you don't them check the horn fuse. Let me know if this works.

If the horn works with direct connection, and fuse, relay, and circuit breaker are all good, it's the switch in the steering wheel. If it is anything like the chrysler auto's, the switch and the air bag module is all one unit, can't replace just the switch, the whole thing, and it's about $700 to $800 for those units, at least it is here, and can only get them at a dealer.